<code id="nzo5t"></code><noscript id="nzo5t"></noscript>
  1. <var id="nzo5t"><label id="nzo5t"></label></var>
    <code id="nzo5t"><cite id="nzo5t"><tr id="nzo5t"></tr></cite></code>
    <input id="nzo5t"><output id="nzo5t"></output></input>
    <table id="nzo5t"></table>
    <table id="nzo5t"></table>
  2. 無憂雅思網_雅思預測_雅思機經_雅思考試_雅思資料下載_雅思名師_2018年雅思考試時間


      Questions 12-20

      The elements other than hydrogen and helium exist In such small quantities that it is accurate to say that the universe somewhat more than 25 percent helium by weight and somewhat less than 25 percent hydrogen.

      Astronomers have measured the abundance of helium throughout our galaxy and in other galaxies as well. Helium has been found In old stars, in relatively young ones, in interstellar gas, and in the distant objects known as quasars. Helium nuclei have also been found to be constituents of cosmic rays that fall on the earth (cosmic "rays" are not really a form of radiation; they consist of rapidly moving particles of numerous different kinds). It doesn't seem to make very much difference where the helium is found. Its relative abundance never seems to vary much. In some places, there may be slightly more of it; In others, slightly less, but the ratio of helium to hydrogen nuclei always remains about the same.

      Helium is created in stars. In fact, nuclear reactions that convert hydrogen to helium are responsible for most of the energy that stars produce. However, the amount of helium that could have been produced in this manner can be calculated, and it turns out to be no more than a few percent. The universe has not existed long enough for this figure to he significantly greater. Consequently, if the universe is somewhat more than 25 percent helium now, then it must have been about 25 percent helium at a time near the beginning..

      However, when the universe was less than one minute old, no helium could have existed. Calculations indicate that before this time temperatures were too high and particles of matter were moving around much too rapidly. It was only after the one-minute point that helium could exist. By this time, the univ

      erse had cooled sufficiently that neutrons and protons could stick together. But the nuclear reactions that led to the formation of helium went on for only a relatively short time. By the time the universe was a few minutes old, helium production had effectively ceased.

      12. what does the passage mainly explain?

      (A)How stars produce energy

      (B)The difference between helium and hydrogen

      (C)When most of the helium in the universe was formed

      (D)Why hydrogen is abundant

      13. According to the passage, helium is

      (A) the second-most abundant element in the universe

      (B) difficult to detect

      (C) the oldest element in the universe

      (D) the most prevalent element in quasars

      14. The word "constituents" in line 7 is closest in meaning to

      (A) relatives

      (B) causes


      (D) targets

      15. Why does the author mention "cosmic rays't' in line 7?

      (A)As part of a list of things containing helium

      (B) As an example of an unsolved astronomical puzzle

      (C) To explain how the universe began

      (D) To explain the abundance of hydrogen in the universe

      16. The word "vary" in line 10 is closest ill meaning to

      (A) mean

      (B) stretch

      (C) change

      (D) include

      17. The creation of helium within stars

      (A) cannot be measured

      (B) produces energy

      (C) produces hydrogen as a by-product

      (D) causes helium to be much more abundant In old stars than In young star:

      18. The word "calculated" in line 15 is closest in meaning to

      (A) ignored

      (B) converted

      (C) increased

      (D) determined

      19. Most of the helium in the universe was formed

      (A) in interstellar space

      (B) in a very short time

      (C) during the first minute of the universe's existence

      (D) before most of the hydrogen

      20. The word "ceased" in line 26 is closest in meaning to



      (C)taken hold


      Questions 21-30  In colonial America, people generally covered their beds with decorative quilts resembling those of the lands from which the quitters had come. Wealthy and socially prominent settlers made quilts of the English type, cut from large lengths of cloth of the same color and texture rather than stitched together from smaller pieces. They mad these until the advent of the Revolutionary War in I 775, when everything English came to be frowned upon.

      Among the whole-cloth quilts made by these wealthy settlers during the early period are.

    成 人 黄 色 网 站 视频 s色,成年片费网站色大全免费观看,成人aⅴ免费视频在线观看,成年片费网站色大全免费观看